This invention relates to cordless telephones, and in particular to digital cordless telephones using a form of spread spectrum communication between a wireless, battery-operated handset and a base station unit.
In selected frequency bands in the United States, wireless transmission is permitted without the need for license if certain standards of signal level and signal dispersion are satisfied. One such band is the Industrial, Scientific, Medical (ISM) Band (902 MHz-928 MHz). In recent years, the ISM band has become of interest for license-free voice and data applications as technology has advanced to take advantage of the availability and propagation characteristics of this band.
One possible technology which could satisfy the requirements of this band is direct sequence spread spectrum (DSSS). In this modulation technique, an information signal is modulated with a pseudo-random noise bit sequence to spread the information signal across a relatively broad spectrum. A digital code sequence can be used for example to modulate a phase shift keyed signal, spreading a narrow band (e.g., 20 kHz) signal over a channel of several Megahertz bandwidth.
Recently, the European Telecommunications Standards Institute has promulgated a standard for digital cordless telephone control and signalling called the Common Air Interface Second Generation Cordless Telephone-type digital signalling protocol. Commonly known as the CT-2 standard, it is designated I-ETS Interim Standard # I-ETS 300 131 of 1993. The CT-2 standard was originally devised to take advantage of available channels in the 864-868 MHz range with a data rate at 72 KPBS per FDMA channel. However, U.S. spectrum allocation standards require that a transmitter using fixed, narrow-frequency modulation techniques in that spectrum obtain an FCC license.
It is neither commercially desirable nor practical to obtain licenses for every cordless telephone in this country. Therefore, the CT-2 standard may not be a practical solution for second-generation cordless telephones in the U.S.A. and probably elsewhere.
Monolithic integrated circuit controller devices are available which accept/produce user input and output directly as voice and/or data and provide/accept digital input and output (serialized binary bit streams) which fully satisfy the CT-2 signalling and protocol standard. It has been presumed that the modulation employed with a CT-2 controller is of a relatively narrow-band digital format, such as Minimum Frequency Shift Keyed (FSK) modulation. These controllers are intended for example to be incorporated into battery-powered cordless handsets, together with a low-powered r.f. transceiver and digital modulator.
A CT-2 controller in integrated circuit form, such as a type Am79C410A controller (Advanced Micro Devices, Sunnyvale, Calif.), typically incorporates an embedded general purpose microcomputer. Upon investigation, it has been determined that such an embedded microcomputer is not fully utilized in its originally-intended application, although it has inadequate capability to handle complex signal processing functions in real time. Moreover, if the controller were in heavy use during "on-hook" idle time, the battery-powered handset would presumably discharge the battery at an unacceptably fast rate for typical applications.
What is needed is a cordless, battery-operated telephone which has relatively low power consumption and yet which is suitable to operate in license-exempt spectrum. It also would be advantageous to employ an established communication protocol.